1. Field of the Invention
The present invention relates to high speed single pass inkjet printing methods exhibiting high image quality.
2. Description of the Related Art
In inkjet printing, tiny drops of ink fluid are projected directly onto an ink-receiver surface without physical contact between the printing device and the ink-receiver. The printing device stores the printing data electronically and controls a mechanism for ejecting the drops image-wise. Printing is accomplished by moving a print head across the ink-receiver or vice versa or both.
In a single pass printing process, usually the ink-jet print heads cover the whole width of the ink-receiver and can thus remain stationary while the ink-receiver surface is transported under the ink-jet printing heads. This allows for high speed printing if good image quality is attainable on a wide variety of ink receivers.
The composition of the inkjet ink is dependent on the inkjet printing method used and on the nature of the ink-receiver to be printed. UV-curable inks are more suitable for non-absorbent ink-receivers than e.g. water or solvent based inkjet inks. However the behaviour and interaction of a UV-curable ink on a substantially non-absorbing ink-receiver was found to be quite complicated compared to water or solvent based inks on absorbent ink-receivers. In particular, a good and controlled spreading of the ink on the ink receiver is problematic.
EP 1199181 A (TOYO INK) discloses a method for ink-jet printing on a surface of a synthetic resin substrate comprising the steps of:
1. conducting a surface treatment to the surface so as to provide the surface with a specific surface free energy of 65-72 mJ/m2 
2. providing an activation energy beam curable ink having a surface tension of 25-40 mN/m
3. discharging the ink onto the surface having the specific surface free energy with an ink-jet printing device thereby forming printed portions of said ink on the surface and
4. projecting an activation energy beam onto the printed portions.
The method of EP 1199181 A (TOYO INK) appears to teach that the surface energy of the ink-receiver surface should be greater than the surface energy of the ink. Yet in the examples, although the surface energy of the four untreated synthetic resin substrates (ABS, PBT, PE and PS) was higher than the surface energy of the four different inks, a good ‘quality of image’ i.e. good spreading of the ink was not observed. The surface treatments used in the examples to increase the surface free energy of the ink receiver were corona treatments and plasma treatments. Since the life-time of such surface treatments is rather limited, it is best to incorporate the surface treatment equipment into the inkjet printer which makes the printer more complex and expensive.
EP 2053104 A (AGFA GRAPHICS) discloses a radiation curable inkjet printing method for producing printed plastic bags using a single pass inkjet printer wherein a primed polymeric substrate has a surface energy Ssub which is at least 4 mN/m smaller than the surface tension SLiq of the non-aqueous radiation curable inkjet liquid.
In general, the surface tension used to characterize an inkjet ink is its “static” surface tension. However, inkjet printing is a dynamic process wherein the surface tension changes dramatically over a time scale measured in tens of milliseconds. Surface active molecules diffuse to and orient themselves on newly formed surfaces at different speeds. Depending on the type of molecule and surrounding medium, they reduce the surface tension at different rates. Such newly formed surfaces include not only the surface of the ink droplet leaving the nozzle of a print head, but also the surface of the ink droplet landing on the ink receiver. The maximum bubble pressure tensiometry is the only technique that allows measurements of dynamic surface tensions of surfactant solutions in the short time range down to milliseconds. A traditional ring or plate tensiometer cannot measure these fast changes.
EP 1645605 A (TETENAL) discloses a radiation-hardenable inkjet ink wherein the dynamic surface tension within the first second has to drop at least 4 mN/m in order to improve the adhesion on a wide variety of substrates. According to paragraph [0026], the dynamic surface tension of the ink measured by maximum bubble pressure tensiometry was 37 mN/m at a surface age of 10 ms and 30 mN/m at a surface age of 1000 ms.
Spreading of a UV curable inkjet ink on an ink receiver can further be controlled by a partial curing or “pin curing” treatment wherein the ink droplet is “pinned”, i.e. immobilized and no further spreading occurs. For example, WO 2004/002746 (INCA) discloses an inkjet printing method of printing an area of a substrate in a plurality of passes using curable ink, the method comprising depositing a first pass of ink on the area; partially curing ink deposited in the first pass; depositing a second pass of ink on the area; and fully curing the ink on the area.
WO 03/074619 (DOTRIX/SERICOL) discloses a single pass inkjet printing process comprising the steps of applying a first ink drop to a substrate and subsequently applying a second ink drop on to the first ink drop without intermediate solidification of the first ink drop, wherein the first and second ink drops have a different viscosity, surface tension or curing speed. In the examples, the use of a high-speed single pass inkjet printer was disclosed for printing UV-curable inks on a PVC substrate by a ‘wet-on-wet printing’ process, wherein the first/subsequent ink drops are not cured, i.e. they are not irradiated prior to application of the next ink drop. In this way an increase in the ink spreading can be realized due to the increased volume of ink of the combined ink drops on the substrate. However, although the spreading of the ink can be increased in this manner, neighbouring drops on the ink-receiver tend to coalescence and bleed into each other, especially on non-absorbing ink-receivers having a small surface energy.
Problems with gloss homogeneity are observed when the printing speed increases, such as e.g. in single pass inkjet printing. EP 1930169 A (AGFA GRAPHICS) discloses a UV-curable inkjet printing method using a first set of printing passes during which partial curing takes place, followed by a second set of passes during which no partial curing takes place for improving the gloss homogeneity.
Therefore it is desirable to be able to print inkjet images, especially on non-absorbing ink-receivers having a small surface energy, by single pass inkjet printing which exhibit sufficient ink spreading without requiring a surface treatment such as corona and while not exhibiting problems of coalescence, bleeding and gloss homogeneity.